A power converter is a power processing circuit that converts an input voltage level into a specified output voltage level. In many applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include at least one power switch, a transformer and a rectifier on a secondary side of the transformer. The rectifier within the converter generates a DC voltage at the output of the converter. Conventionally, the rectifier includes a plurality of rectifying diodes that conduct the load current only when forward-biased in response to an input waveform to the rectifier. However, diodes produce a voltage drop thereacross when forward-biased. Given an escalating requirement for a more compact converter that delivers a lower output voltage (e.g., 3.3 volts for a central processing unit of a computer), it is highly desirable to avoid the voltage drop inherent in the rectifying diodes and thereby increase the efficiency of the converter.
A more efficient rectifier can be attained in converters by replacing the rectifying diodes with active switches, such as field effect transistors. The switches are periodically toggled between conducting and non-conducting states in synchronization with the periodic waveform to be rectified. A rectifier employing active switches is conventionally referred to as a synchronous rectifier. For a better understanding of synchronous rectifiers and the control thereof, see U.S. Pat. No. 5,956,245 entitled “Circuit and Method for Controlling a Synchronous Rectifier Converter,” issued Sep. 21, 1999 to Rozman, which is incorporated herein by reference.
In low to medium level power applications (e.g., 30 to 800 watts), a forward converter topology is widely used. A DC/DC forward converter generally includes a transformer, a power switch on a primary side of the transformer, and a rectifier and output filter on a secondary side of the transformer. The power switch, coupled in series with a primary winding of the transformer, converts an input DC voltage into an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates therefrom a desired DC voltage that is filtered by the output filter at an output of the forward converter.
A practical concern regarding forward converters, and power converters in general, is that a magnetizing current of the transformer should be taken into account during the design of the converter. Otherwise, the magnetic energy stored in a core of the transformer by the magnetizing current may cause a failure in the converter. One approach for dealing with the magnetic energy (and reducing the deleterious effects associated therewith) is to introduce an active clamp circuit across the power switch of the forward converter.
A conventional active clamp circuit includes a series-coupled clamp switch and clamp capacitor, coupled across the power switch, that clamps a voltage across the windings of the transformer when the power switch is not conducting. When the power switch is transitioned to a non-conducting state, the clamp switch conducts to recover the magnetic energy stored in the core of the transformer to the clamp capacitor. As a result, the magnetic energy is dissipated to allow a reset of the core of the transformer. For additional information about clamp circuits and the benefits associated therewith, see U.S. Pat. No. 5,126,931 entitled “Fixed Frequency Single Ended Forward Converter Switching at Zero Voltage,” issued Jun. 30, 1992 to Jitaru, and U.S. Pat. No. 5,303,138 entitled “Low Loss Synchronous Rectifier for Application to Clamped-mode Power Converters,” issued Apr. 12, 1994 to Rozman, which are incorporated herein by reference.
Thus, when employing an active clamp in a forward converter, the power switch conducts for a primary duty cycle D to impress the DC input voltage across the primary winding of the transformer. The power switch is then transitioned to a non-conducting state and the clamp switch conducts for a complementary duty cycle 1-D to allow the active clamp circuit to reset the transformer. As described in U.S. Pat. No. RE 36,098 entitled “Optimal Resetting of the Transformer's Core in Single-Ended Forward Converters,” issued Feb. 16, 1999 to Vinciarelli, which is incorporated herein by reference, it is preferable to introduce a delay between conduction periods of the power switch and the clamp switch of the forward converter.
A delay between the conduction periods of the power switch and clamp switch substantially forecloses an opportunity for cross current conduction therebetween. As clearly understood by those skilled in the art, cross current conduction between the power switch and clamp switch may cause a sharp rise in a current in the forward converter thereby leading to a potential failure of components therein. Additionally, in forward converters employing a synchronous rectifier having a pair of complementary synchronous rectifier switches on the secondary side of the transformer, incorporating the delay further reduces the probability of cross current conduction between the synchronous rectifier switches thereof.
Conversely, it is important to maintain as small a delay between the conduction intervals of the power switch and clamp switch as is practical inasmuch as the forward converter may not be processing energy during the period of delay resulting in a less efficient converter. Additionally, in forward converters employing the complementary pair of synchronous rectifier switches, a body diode of the synchronous rectifier switch being transitioned to a non-conducting state may conduct when the period of delay is excessive thereby decreasing the efficiency of the converter. Therefore, a controller for the power switch and the clamp switch is predisposed to incorporate a definite, but small, delay between the non-conducting state of the power switch and the conducting state of the clamp switch, and vice versa.
In many instances, however, the delay between the conduction periods of complementary switches in the power converter has been static. In other words, a designer of the power converter predetermines the period of delay for selected operating conditions and the period of delay remains the same notwithstanding the true operating conditions of the power converter. A static delay, however, in view of the variable operating conditions of the power converter can be problematic.
There have been attempts proposed in the past to vary drive signals to selected switches in a power converter. One system of providing a variable drive signal to switches in a power converter was introduced in U.S. Pat. No. RE 37,221 entitled “Power Converter Adaptively Driven,” issued Jun. 12, 2001 to Bowman, et al. (“Bowman”), which is incorporated herein by reference. Bowman recognized that a delay introduced in a drive waveform between the inverter (i.e, the power switch) and synchronous rectifier of the power converter should not be static, but rather should be variable. Bowman, therefore, introduced a variable nonconcurrent change in the state of the inverter and the synchronous rectifier according to a function of operating conditions of the power converter. While Bowman provides a variable delay between the inverter and synchronous rectifier of a power converter, the reference does not address a delay between two complementary switches in a power converter such as a forward converter employing an active clamp.
Accordingly, what is needed in the art is a system and method capable of altering a drive signal to at least one of a pair of complementary switches of a power converter thereby modifying a delay between conduction periods thereof based upon selected operating parameters associated with the power converter.